Problem: Solve for $n$ : $-9 = -29 + n$
Answer: Add $29$ to both sides: $ \begin{eqnarray} \\ \hphantom{-9 {+ 29}} &\hphantom{=}& \hphantom{n} \\ -9 &=& -29 + n \\ \\ {+29} && {+29} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -9 {+ 29} &=& n \\ \end{eqnarray} $ Simplify. $ n = 20$